There exists a great variety of methods and apparatuses used in the industry for measuring thickness of coating films and layers applied or laid onto substrates. These methods and apparatuses can be classified in accordance with different criteria. Classification of one type divides these methods into direct and indirect.
An example of the direct method is measurement of a thickness in thin metal coating films by means of so-called X-ray reflectivity. One of these methods is based on a principle that X-rays and gamma rays are absorbed by matter. When a beam of rays passes through a material, the amount of the beam absorbed depends on what elements the material consists of, and on how much of the material the beam has to pass through. This phenomenon is used to measure the thickness or density of a material. The advantage of measuring in this way is that the gauge does not have to touch the material it is measuring. In other words, in thickness measurement, the surface of a web or strip product will not be scratched. The instrument for this method is e.g., RMS1000 Radiometric System produced by Staplethorne Ltd (UK). The instrument uses a suitable radiation source and one or more radiation detectors installed in a mechanical housing which also provides high quality radiological shielding. The source may be an X-ray tube or a radioactive source. The instrument also uses a set of beam defining collimators and one or more radiation detectors. The detectors measure the radiation absorbed within the object or flow being measured and output of the signal data to a computer. For thickness gauging, the collimators usually define a single, narrow beam. This gives optimum spatial resolution. A disadvantage of radiation methods is the use of X-ray or gamma radiation that requires special safety measures for protection of the users against the radiation. The instruments of this type are the most expensive as compared to metrological equipment of other systems.
Another example of direct measurement is a method of optical interferometry, described e.g., by I. Herman in “Optical Diagnostics for Thin Film Processing”, Academic Press, 1996, Chapter 9. Although the optical interferometry method produces the most accurate results in measuring the thickness of a coating film, it has a limitation. More specifically, for conductive films, to which the present invention pertains, this method is limited to measurement of extremely thin coating films which are thin to the extent that a nontransparent material, such as metal, functions as transparent. In other words, this method is unsuitable or is difficult to use for measuring conductive films thicker than 200 Å to 500 Å.
Another example of direct measurement methods is measuring thickness of a film in situ in the course of its formation, e.g., in sputtering, magnetron target sputtering, CVD, PVD, etc. These methods, which are also described in the aforementioned book of I. Herman, may involve the use of the aforementioned optical interferometry or ellipsometry. However, in this case measurement is carried out with reference to both the surface of the substrate and the surface of the growing layer. Therefore, this method is inapplicable to measuring thickness of the film that has been already deposited.
In view of the problems associated with direct methods, indirect non-destructive methods are more popular for measuring thickness of ready-made films. An example of a well-known non-destructive indirect method used for measuring thickness of a film is the so-called “four-point probe method”. This method is based on the use of four contacts, which are brought into physical contact with the surface of the film being measured. As a rule, all four contacts are equally spaced and arranged in line, although this is not a compulsory requirement. Detailed description of the four-point probe method can be found in “Semiconductor Material and Device Characterization” John Wiley & Sons, Inc., N.Y., 1990, pp. 2-40, by D. Schroder. The same book describes how to interpret the results of measurements. This method is classified as indirect because the results of measurement are indirectly related to the thickness of the film. It is understood that each measurement of electric characteristics has to correlated with the actual thickness of the film in each particular measurement, e.g., by cutting a sample from the object and measuring the thickness of the film in a cross-section of the sample, e.g., with the use of an optical or electron microscope. Nevertheless, in view of its simplicity, low cost, and convenience of handling, the four-point probe method is the most popular in the semiconductor industry.
However, the four-point method has some disadvantages. The main problem associated with the aforementioned four-point probe method is that in each measurement it is required to ensure reliable contact in each measurement point. This is difficult to achieve since conditions of contact vary from sample to sample as well as between the four pointed contact elements of the probe itself in repeated measurement with the same probe. Such non-uniformity affects the results of measurements and makes it impossible to perform precision calibration.
Known in the art are also methods for measuring film thickness with the use of inductive sensors. For example, U.S. Pat. No. 6,072,313 issued in 2000 to L. Li et al. describes in-situ monitoring and control of conductive films by detecting changes in induced eddy currents. More specifically, the change in thickness of a film on an underlying body such as a semiconductor substrate is monitored in situ by inducing a current in the film, and as the thickness of the film changes (either increase or decrease), the changes in the current are detected. With a conductive film, eddy currents are induced in the film by generating an alternating electromagnetic field with a sensor, which includes a capacitor and an inductor. The main idea of the apparatus of U.S. Pat. No. 6,072,313 consists in using a resistor and a capacitor in a parallel resonance circuit. The resonance is caused by means of an oscillator. The inductive coupling between the oscillation circuit and the Eddy current inducted in the coating is used for improving a signal/noise ratio and can be used for improving quality of measurements. In fact, this is a method well known in the radioelectronics for measuring under conditions of the electrical resonance. The above patent describes the aforementioned inductive method for measuring thickness of a film in chemical mechanical polishing (CMP).
A similar inductive method, which was used for measuring thickness of a slag, is disclosed in U.S. Pat. No. 5,781,008 issued in 1998 to J. Muller et al. The invention relates to an apparatus for measuring the thickness of a slag layer on a metal melt in a metallurgical vessel. The apparatus comprises a first inductive eddy current sensor which indicates the distance of the apparatus from the metal melt as it is moved toward the melt. A second sensor detects when the apparatus reaches a predetermined distance relative to or contacts the slag layer and triggers the inductive eddy-current sensor when such distance is attained. The sensors are arranged in a predetermined spatial relation, and the thickness of the slag layer is determined by an evaluation device, which analyzes the received signals. The apparatus permits measurement of the thickness of the slag layer without the need of additional equipment (e.g. mechanical lance movement or distance measurement).
The method and apparatus of U.S. Pat. No. 5,781,008 relate to macro-measurements of thick layers, and the sensors used in the apparatus of this invention are inapplicable for measuring thickness of thin-film coatings on such objects as semiconductor wafers and hard-drive disks. Furthermore, once the second sensor has detected that the apparatus reached a predetermined distance relative to or contacts the slag layer, this distance remains unchanged during the measurement procedure. This condition is unacceptable for measuring thickness of a thin film with microscopic thickness, which moves relative to the sensor, e.g., for mapping, i.e., for determining deviations of the thickness over the substrate.
In order to understand why the use of known eddy-current sensor systems utilizing a measurement eddy-current sensor and a proximity sensor cannot be easily and directly applicable to measurement of microscopically-thin film coatings on conductive or non-conductive substrates, let us consider constructions and operations of the aforementioned known systems in more detail.
Generally speaking, all inductive sensors are based on the principle that in its simplest form an inductive sensor comprises a conductive coil, which is located in close proximity to a conductive film to be measured and in which an electric current is induced. The conductive film can be considered as a short-circuited virtual coil turn with a predetermined electrical resistance. Since a mutual inductance exists between the aforementioned conductive coil and the virtual coil turn, an electric current is generated in the virtual coil turn. This current is known as eddy current or Foucault current. Resistance of the virtual coil turn, which depends on the material of the conductive film and, naturally, on its thickness, influences the amplitude of the alternating current induced in the virtual turn. It is understood that the amplitude of the aforementioned current will depend also on the thickness of the conductive film.
However, realization of a method and apparatus based on the above principle in application to thin films is not obvious. This is because such realization would involve a number of important variable parameters which depend on a specific mode of realization and which are interrelated so that their relationships not always can be realized in a practical device.
In order to substantiate the above statement, let us consider the construction of an inductive sensor of the aforementioned type in more detail.
FIG. 1 is a schematic view of a known inductive sensor used, e.g., for positioning of an inductive sensor 22′ relative to the surface S′ of an object 24′. Let us assume that the surface S′ of the object 24′ is conductive. The inductive sensor comprises an electromagnetic coil 26′ connected to an electronic unit 28′, which, in turn, is connected to a signal processing unit 30′. The latter can be connected, e.g., to a computer (not shown). The electronic unit 28′ may contain a signal oscillator (not shown), which induces in the electromagnetic coil 26′ an alternating current with a frequency within the range from several kHz to several hundred KHz. In FIG. 1, symbol D designates the distance between the electromagnetic coil 26′ and the surface S.
In a simplified form, the sensor of FIG. 1 can be represented by a model shown in FIG. 2. In this model, L1 designates inductance of the electromagnetic coil 26′; R1 designates resistance of the coil 26′; L2 designates inductance of the aforementioned virtual coil turn 27′ (FIG. 1); and R2 is electrical resistance of the aforementioned virtual coil turn 27′. M designates mutual induction between L1 and L2.
It can be seen from the model of FIG. 2 that the amplitude of current I generated in coil 26′ will depend on R1, L1, L2, R2 and M. It is also understood that in this influence M is the most important parameter since it directly depends on the distance D (FIG. 1) from the inductive sensor 22′ to the surface S.
FIG. 3 is further simplification of the model of FIG. 2. Parameters L and R are functions that can be expressed in terms of L1, L2, M, R1, and R2. Therefore, as shown in FIG. 3, these parameters can be considered as functions L(D) and R(D), where D is the aforementioned distance (FIG. 1).
The model of FIG. 3 can also be characterized by a quality factor Q, which is directly proportional to the frequency of the current in the sensor coil 26′, to inductance of the sensor of FIG. 3, and is inversely proportional to a distance D (FIG. 1) from the sensor coil 26′ to the surface S. The higher is the value of Q, the higher is stability of the measurement system and the higher is the measuring accuracy. Thus it is clear that in order to achieve a higher value of Q, it is necessary to operate on higher frequencies of the alternating currents in the inductance coil 26′. Analysis of relationships between Q, L, and R for a fixed distance D was made by S. Roach in article “Designing and Building an Eddy Current Position Sensor” at http://www.sensormag.com/articles/0998/ edd0998/main.shtml. S. Roach introduces an important parameter, i.e., a ratio of D to the diameter of the sensor coil 26′, and shows that R does not practically depend on the above ratio, while the increase of this parameter leads to the growth in L and Q. When distance D becomes equal approximately to the diameter of the coil 26′, all three parameters, i.e., L, Q, and R are stabilized, i.e., further increase in the distance practically does not change these parameters. In his important work, S. Roach generalized the relationships between the aforementioned parameters and showed that, irrespective of actual dimensions of the sensor, “the rapid loss of sensitivity with distance strictly limits the range of eddy current sensor to about ½ the coil diameter and constitutes the most important limitation of this type of sensing”.
The impedance of the coil also depends on such factors as film thickness, flatness of the film, transverse dimensions, temperature of the film and coil, coil geometry and DC resistance, operating frequency, magnetic and electric properties of the film, etc.
As far as the operating frequency of the inductive coil is concerned, the sensor possesses a self-resonance frequency, which is generated by an oscillating circuit formed by the power-supply cable and the capacitor. As has been shown by S. Roach, in order to improve sensitivity, it is recommended to increase the quality factor Q and hence the frequency. However, the sensor must operate on frequencies at least a factor of three below the self-resonant frequency. Thus, practical frequency values for air core coils typically lie between 10 kHz and 10 MHz.
The depth of penetration of the electromagnetic field into the conductive film is also important for understanding the principle of operation of an inductive sensor. It is known that when an alternating electromagnetic field propagates from non-conductive medium into a conductive medium, it is dampened according to an exponential law. For the case of propagation through the flat interface, electric and magnetic components of the alternating electromagnetic field can be expressed by the following formulae:E=E0exp(−αx)H=H0exp(−αx),where α=(πfμoμσDC)1/2, f is oscillation frequency of the electromagnetic field, σDC is conductivity of the medium measured on direct current, and μ0=1.26×10−6 H/m (for non-magnetic materials μ=1). Distance x from the interface, which is equal tox=δ=1/α=1/(πfμoσDC)1/2  (1a)and at which the amplitude of the electromagnetic wave decreases by e times, is called the depth of penetration or a skin layer thickness. Based on formula (1a), for copper on frequency of 10 kHz the skin depth δ is equal approximately to 650 μm, on frequency of 100 kHz to 200 μm, on frequency of 1 MHz to 65 μm, on frequency of 10 MHz to 20 μm, on frequency 100 MHz to 6.5 μm, and on frequency 10 GHz to 0.65 μm.
The above values show that for the films used in the semiconductor industry, which are typically with the thickness on the order of 1 μm or thinner, the electromagnetic field can be considered practically as uniform. This is because on any frequency in the range from 10 KHz to 10 MHz the electromagnetic waves begin to dampen on much greater depth than the thickness of the aforementioned films. It is only on frequencies substantially greater than 100 MHz (e.g., 10 GHz), the depth of penetration of the electromagnetic fields becomes comparable with the thickness of the film.
Similar trend is observed in the films made from other metals, where the skin layer is even thicker because of lower conductivity. At the same time, deviations from uniformity in the thickness of the conductive coating films used in the semiconductor industry, e.g., copper or aluminum layers on the surface of silicon substrates, should not exceed 5%, and in some cases 2% of the average thickness of the layer. In other words, the deviations should be measured in hundreds of Angstroms. It is understood that conventional inductive sensors of the types described above and used in a conventional manner are inapplicable for the solution of the above problem. Furthermore, in order to match conditions of semiconductor production, such sensors must have miniature constructions in order to be installed in close proximity to the measurement site. The distance between the measurement element of the inductive sensor and the surface of the film being measured also becomes a critical issue. Due to high sensitivity, the sensor becomes very sensitive to the influence of the environment, especially, mechanical vibrations, variations in temperature, etc.
In their previous U.S. patent application Ser. No. 09/954,550 filed on Sep. 17, 2001, the applicants made an attempt to solve the above problems by developing a method and apparatus for measuring thickness and deviations from the thickness of thin conductive coatings on various substrates, e.g., metal coating films on semiconductor wafers or hard drive disks. The thickness of the films may be as small as fractions of microns. The apparatus consists of an inductive sensor and a proximity sensor, which are rigidly interconnected though a piezo-actuator used for displacements of the inductive sensor with respect to the surface of the object being measured. Based on the results of the operation of the proximity sensor, the inductive sensor is maintained at a constant distance from the controlled surface. Variations in the thickness of the coating film and in the distance between the inductive sensor and the coating film change the current in the inductive coil of the sensor. The inductive sensor is calibrated so that, for a predetermined object with a predetermined metal coating and thickness of the coating, variations in the amplitude of the inductive sensor current reflect fluctuations in the thickness of the coating. The distinguishing feature of the invention resides in the actuating mechanism of microdisplacements and in the measurement and control units that realize interconnection between the proximity sensor and the inductive sensor via the actuating mechanism. The actuating mechanism is a piezo actuator. Measurement of the film thickness in the submicron range becomes possible due to highly accurate dynamic stabilization of the aforementioned distance between the inductive sensor and the object. According to one embodiment, the distance is controlled optically with the use of a miniature interferometer, which is rigidly connected to the inductive sensor. According to another embodiment, the distance is controlled with the use of a capacitance sensor, which is also rigidly connected to the inductive sensor.
However, the sensor disclosed in the aforementioned patent application could not completely solve the problems associated with accurate measurement of super-thin films, e.g., of those thinner than 500 Angstroms. This is because the construction of the aforementioned sensor is limited with regard to the range of operation frequencies, i.e., the sensor cannot be use in frequencies exceeding 30 MHz. Furthermore, the aforementioned sensor requires the use of a complicated distance-stabilization system. The above problems restrict practical application of the method and apparatus of U.S. patent application Ser. No. 09/954,550 for measuring thickness of very thin films and deviations from the thickness in the aforementioned films. Furthermore, it is obvious that the aforementioned method and apparatus do not allow thickness measurement of non-conductive films. The sensor has relatively large overall dimensions and comprises a stationary measurement instrument.
Another disadvantage of the sensor of the aforementioned application is that it is very sensitive to the distance between the sensor and the film. This requirement dictates the use of expensive and complicated distance-measurement means such as microinterferometers or microscopes.
In order to solve the above problems, the applicants have developed a new apparatus and method for measuring thickness and deviations from the thickness of very thin conductive coatings on various non-conductive substrates, or of very thin non-conductive coatings on conductive substrates. These apparatus and method are disclosed in U.S. Pat. No. 6,815,958 issued on Nov. 9, 2004. The apparatus consists of an inductive coil having specific parameters, an external AC generator operating on frequencies, e.g., from 50 MHz to 2.5 GHz, preferably from 100 MHz to 200 MHz, and a measuring instrument, such as an oscilloscope, voltmeter, etc. for measuring output of the sensor. The coil has miniature dimensions. The invention is based on the principle that inductive coil of the sensor, active resistance of the coil winding, inherent capacitance of the inductive coil (or a separate capacitor built into the sensor's circuit), and the aforementioned AC generator form a parallel oscillating circuit. The apparatus operates on very high resonance frequencies, preferably within the range of 100 to 200 MHz, at which a capacitive coupling is established between the coil of the oscillating circuit and the thin films being measured. By measuring the parameters of the resonance oscillating circuit, it becomes possible to measure film thickness below 500 Angstroms.
Although the method and apparatus disclosed in U.S. Pat. No. 6,815,958 make it possible to measure thickness of very thin films, the aforementioned capacitive coupling can still be established in a limited range of frequencies. Furthermore, rapid development of semiconductor technology demands that the accuracy of measurement be even higher than the one attainable with the method and apparatus described in the aforementioned patent application.
It is understood that for measuring a 1000 Angstroms-thick film, even with 5% accuracy, the sensor should depict thickness deviations at least within 50 Angstroms. It is also understood that improvement in the accuracy of measurement of superthin film with specific electromagnetic sensors can be achieved by increasing the frequency of electromagnetic radiation (up to microwave frequency) utilized by such sensors for testing. An example of such a sensor is the one disclosed in U.S. Pat. No. 5,334,941 issued in 1994 to R. King. Although the aforementioned sensor is described in application for measuring complex impedance characteristics of the surface, one can assume that such a resonance sensor can be used also for measuring thickness of thin films. The sensor system consists of a microwave source with a frequency that can be swept within a limited range, a radio-reflectometer bridge, a microwaveguide line that connects the reflectometer bridge with the resonance sensor, and a ratio meter or scalar network analyzer connected to the microwave source and the radio-reflectometer bridge for visual presentation and data monitoring of the test operation. The sensor has a special slit-like structure, which comprises a metal microstrip dipole resonator etched on the surface of a dielectric substrate, which is bonded to a copper ground. This dipole resonator is electromagnetically driven by mutual inductive coupling to a short nonresonant feed slot formed in the ground plane. The slot is driven by a coaxial feed line or a microstrip feed line extending from a swept microwave frequency source, which excites the incident wave. Measurement of the resonant frequency and input coupling factor determines small changes in real and imaginary parts of the dielectric constant or in conductivity for highly conductive materials, such as metals.
In spite of the fact that the microwave reflection resonator sensor described in U.S. Pat. No. 5,334,941 is a device which after a special calibration procedure may appear to be highly efficient and accurate for measuring sufficiently thin films (down to 1000 Angstroms), this sensor may operate only in physical contact with the test object, which is not always permitted in the semiconductor industry where the use of contactless test sensors is preferable.
Thus, an inexpensive, simple, high-accurate and efficient contactless microwave resonance sensor system suitable for measuring film thicknesses below 1000 Angstroms is unknown.